Which of the following masks, when used within a Class A network, would supply enough subnet bits to support 8 subnets?

(A) /10
(B) /11
(C) /9
(D) /8

/11 is the correct answer.

The mask in binary define a number of binary 1s, and the number of binary 1s defines the length of the prefix (network + subnet) part. With a Class A network, the network part is 8 bits. To support 8 subnets, the subnet part must be at least 3 bits long. Two subnet bits would supply only 22 = 4 subnets, while 3 subnet bits supply 23 = 8 subnets. The /11 answer supplies 3 subnet bits.